Phytohormones assignment Plant Hormones

Phytohormones assignment

Plant Hormones: (Phytohormone)

Definition

Certain chemical produced by plants have profound effect on their subsequent growth and development. Such chemicals are called Plant Hormones or Phytohormone.
Phytohormone are synthesized by plants in minute concentration and exert their effect by activating gene expression or inhibiting enzyme or changing properties of membrane.

Types of Phytohormone

There are five kinds of plant hormones
1. Auxins
2. Gibberellins
3. Cytokinins
4. Abscisic Acid
5. Ethene

1. Auxins 

Discovery 

the first auxin was discovered by Fret Went in 1926.

Chemical Nature 

Indol Acetic Acid (I.A.A)
Indol Acetic Acid (I.B.A)
Nephthalene Acetic Acid (N.A.A)
Site of Synthesis
It is synthesize at the apices of stem and foot, young leaves and young embryo.

Role of Auxin 

i. Cell division and cell enlargement
It stimulate teh cell division and cell enlargement and plant in increase the length of plant.
ii. Initiation of Root
Auxins also initiates development of adventitious roots when applied at the cut base of stem.
iii. Abscission
In mature leaves and fruits when auxin production diminishes, a layer of thin walled cells is formed at the base of petiole and stake of fruit. This layer is called Abscission layer and causes fall of leaves and fruit with slight jerk. 

iv. Growth of FruitAuxins produced in young embryo promotes the growth of fruit.
v. ParthenocarpyUse of auxin helps in producing parthenocarpic or seedless fruits. 
vi. Apical DominanceBesides growth promoting function on Auxin, also has inhibitory effect on growth. Growth of apical bud inhibits growth of lateral buds beneath the stem. This phenomenon is termed as apical dominance removal of apical buds initiates growth of lateral buds with more leaves and axillary bud.
vii. WeedicideAuxins are selective weed killer 2-4 dichlorophenoxy acetic acid (-2-4-D) is used to kill weeds in lawn's and cereal crops.

2. Gibberellins 

DiscoveryGibberellins was discovered by T.Yabuta and I.Hayashu in a fungus called Gibberellins funjikuroi. This fungus causes foolish seedling (Bakanae) disease in rice. In this disease the infected rice seedling elongated and ultimately fallover without producing grains.
Chemical NatureThe chemical nature of Gibberellins is Gibberellins acid 70 types of gibberellins have been discovered.

Role of Gibberellins
i. Cell division and cell enlargementLike auxins Gibberellin also promotes cell division and elongation.
ii. Control of DwarfismGibberellins can control genetic and physiological dwartism plants.
iii. Seed GerminationThey promote the synthesis of a-amylase enzyme is dorman seeds due to the production of this enzyme, the seed starts germination.
iv. ParthenocarpyThese hormones help in the formation of seedless fruit which are called Parthenocarpic fruits.
v. Increase of Crop YieldThe crop yield of sugar can can be increased by the application of gibberellin about 50 tons/ acre.
vi. Formation of Flower and Growth of Pollen TubeThey stimulate flowering and the growth of pollen tubes during fertilization

3. Cytokinins 

DiscoveryCytokinins are discovered by Miller in coconut milk.
Chemical NatureChemically there are two types of cytokinins. 
Kinetin It is found in coconut milk etc.
Zeatin It is found in maize.

Role of Cytokinins
i. Cell DivisionThey initiate rapid cell division only in the presence of auxin.
ii. Delay in SenescenceThey also caused delayed senescence (old age).
iii. Breaking of Seed DormancyThey break seed dormancy and promote fruit development some species.

4. Abscisic Acid (A.B.A) 

In contrast to growth promoting hormones, abscisic acid is growth inhibitor, produced by plants during adverse environment conditions such as drought conditions.

Role of Abscisic Acid 

• It increases dormancy in buds and seeds.

• It causes stomata to close.

• It turn leaf primordia into scale.

5. Ethene 

It is a gas which also acts as a growth inhibitor.

Role of Ethene 

• It triggers ripening of fruits.

• It contributes in leaf abscission and also breaks the dormancy of seeds and buds.

It also initiates flowering in plants e.g. pineapple.